The invention relates to a drive hub for a vehicle, particularly a bicycle, with a friction gear the transmission ratio of which can be adjusted in stepless manner.
From the manual "Bicycling Science" by Frank Rowland Witt, David Gordon Wilson, The MIT-Press, Cambridge and London, 1988, 2nd edition, page 282, several construction principles of friction gears are known, reference being made to the problems in connection with such friction gears in connection with bicycle hubs. These problems consist in that in bicycle hubs relatively high torques for low speeds are to be transmitted, in connection with which the transmission ratio is to be adjustable in such a manner that, according to requirements, the hub shell rotates faster or slower than the driver. One of the main constructional forms represented is a friction gear in the form of a ball friction gear, wherein friction balls, stationarily mounted with angle-adjustable ball axes on the outer periphery designed in the manner of a cone, roll both on a driving wheel and on a driven wheel for the transmission of a corresponding torque from the driving plate to the driven plate.
DE-21 36 243 A1 shows a static gear with input shaft and an output shaft coaxial therewith. The friction balls used here are freely rotatably mounted and roll on a total of four locating faces adjustable relative to one another, the axis of rotation of the balls adjusting itself accordingly. The drive of the friction balls is effected over a slanting outer peripheral surface of a drive plate connected with the driving axle. Provided is an expanding coupling for increasing the pressing force as the torque increases. The construction of this gear is costly. A direct precise setting of a desired transmission ratio is not possible. The stationary housing, in which, inter alia, a driving worm of the control device is provided, cannot be replaced by a rotating hub shell.
U.S. Pat. No. 4,735,430 shows a drive hub for bicycles with a friction gear which is based upon the principle of torque transmission by means of friction wheels which roll on toroid surfaces of oppositely placed toroid plates. The hub axle here, differently from the standard drive hubs, is rotatably mounted in the bicycle frame, so that special mounting means are required. The two toroid plates rotate in opposite directions. In order, however, to drive the hub shell in the direction of rotation of the driver, a planetary gear reversing the direction of rotation with stationary planetary gear carrier is therefore necessary in this construction. The planet wheel engage into a ring geared wheel connected with the friction gear, as well as into a sun wheel gearing connected with the hub shell, so that the friction gear therefore runs more slowly than the hub shell. For a predetermined drive performance the friction gear must therefore be designed for the transmission of high torques. Particularly, high pressing forces are necessary in the friction gear so as to avoid undesirable slip. The friction gear is always moving for all possible transmission rations, which brings about friction losses.
An improvement of this hub is shown in EP 0 432 742 A1. The hub axle, as is usual in bicycles, can be incorporated rigidly into the bicycle frame. This bicycle hub also operates according to the principle of toroid plates. For the adaptation of the speed range of this hub to the transmission ratios usual in bicycles a planetary gear is provided, the planet wheel carrier being connected with the hub shell, and the planet wheels engage, on the one hand, into a stationary ring gear, and on the other hand, into a sun wheel connected with the friction gear. The ring gear is connected with the hub axle by means of an inner barrel surrounding the friction gear and extending over a great part of the axial length of the hub shell, which increases the constructional expenditure as well as the required space for incorporation. The planetary gear moves independently of the respective set transmission ratio. Finally, the constructional expenditure for bicycle hubs which operate according to the toroid plate principle is particularly high, especially because the making of the gear demands high manufacturing precision. Also, the transmission range in this construction is confined within relatively narrow limits.